Electricity is a form of energy arising from the interaction of electrically charged particles. Most of the electricity used is in the form of flowing negatively-charged electrons in metal wires, known as electric current. There are two basic types of electric current. Direct current (DC) always flows in the same direction whereas alternating current (AC) regularly reverses the direction of flow. The magnitude of electric current is measured in amperes and the potential for current between two points is measured in volts. Electric current is frequently converted from one type to the other. Devices that convert direct current to alternating current are known as inverters and devices that convert alternating current to direct current are known as rectifiers. Alternating current is much more widely used than direct current because it can be transmitted with less loss and because its voltage can be easily and efficiently increased or decreased in devices known as transformers.
Industrialized countries have vast, interconnected power grids for transmitting alternating current. The voltage is typically greater than 500,000 volts in major transmission lines, about 2,000 to 10,000 volts in local grids, and about 100 to 240 volts for residential use. The alternating current typically has a frequency of either 50 or 60 hertz, i.e., it completes a cycle 50 or 60 times each second. For example, most consumers in the United States receive electricity at 120 volts and 60 hertz whereas most consumers in Europe receive electricity at 230 volts and 50 hertz.
Electricity and magnetism are related forces and both generate force fields that can affect other objects even without direct contact. For example, a moving electric field produces magnetism in nearby magnetic materials and a moving magnetic field produces electrical current in nearby conductive materials. Electricity is commonly created in generators (sometimes known as dynamoelectrics or dynamos) by rotating a magnetic field around metal wire so that current is induced (generated) in the wire. The part of the generator that remains stationary is commonly known as the stator and the part of the generator that rotates is commonly known as the rotor. Generators can produce either direct current or alternating current. In an alternating current generator, the speed at which the rotor rotates determines the frequency of the alternating current produced.
A variety of mechanisms are used to move the rotor in a generator. For example, small household generators typically use an internal combustion engine to turn the rotor. Larger generators use a turbine consisting of a series of blades on a shaft. The blades are contacted by a flowing fluid which rotates the turbine shaft which, in turn, rotates the rotor in the generator. Hydroelectric generators at large dams use turbines that are rotated by falling water. Most power plants use turbines that are rotated by flowing steam, which is produced by burning a fuel (coal, gas, oil, etc.) or by conducting a nuclear reaction. Generators can also be powered by turbines that are rotated by wind. Producing electricity using wind turbines has many environmental advantages and is becoming increasingly popular. For example, approximately twenty percent of all electrical power in some northern European countries is produced from wind turbines.
A wind turbine generator typically includes a turbine consisting of three or four large blades mounted on a variable pitch hub. The variable pitch hub enables the desired rotational speed to be obtained over a wider range of wind velocities. The hub is connected to a shaft that is, in turn, connected to a step-up gearbox that increases the rotational speed by as much as one hundred times to achieve the desired frequency of the alternating current. The output shaft from the gearbox then turns a conventional alternating current generator. The phrase “wind turbine generator” is used herein to refer to the generator itself and the phrase “wind turbine generator system” is used to refer to the generator system (including the generator, turbine, and linkage between the generator and turbine) unless the context indicates otherwise.
A large number of wind turbine generators and wind turbine generator systems have been disclosed, including those in Kirschbaum, U.S. Pat. No. 4,291,233, Sep. 22, 1981; Appel, U.S. Pat. No. 4,606,697, Aug. 19, 1986; Kollitz et al., U.S. Pat. No. 5,375,968, Dec. 27, 1994; and Mikhail et al., U.S. Pat. No. 6,137,187, Oct. 24, 2000.
Conventional wind turbine generators and wind turbine generator systems suffer from several disadvantages. One disadvantage of the systems is that the frictional losses in a step-up gearbox reduce the energy transmitted to the generator. Approximately twenty percent of the energy captured by the wind turbine is lost in the gearbox. The heat generated in the gearbox often requires a separate oil cooler and fan. Another disadvantage is that the gearbox, cooler, and fan contain many moving parts which require maintenance and are prone to failure.
Accordingly, there is a demand for an improved wind turbine generator and wind turbine generator system. More particularly, there is a demand for a generator that is directly driven by the wind turbine. There is also a demand for a system that eliminates the gearbox and that has an overall efficiency of greater than about ninety-five percent.